Improve and extract username and email generators into a class

What does this MR do and why?

See this thread !112931 (comment 1295900394).

We need the same username and email generators for a service class that will create Service Account, #392000 (closed).

This MR extracts the functionality to Gitlab::Utils::UsernameAndEmailGenerator class to share it between classes by using the Composition.

This MR also improves username/email generators by

• changing {random_string} part from SecureRandom.hex(8) to SecureRandom.hex(16) to reduce the probability(1/16**16) of duplicates, see !112548 (comment 1289885385).
• Returns uniquify conflict resolver back in case of duplicates. Though there is a very low probability 1/16*16, technically, there could be conflicts and this will guard against "username/email reserved" issues.
• Addresses all issues mentioned in !112931 (comment 1299350132) and adds tests for each issue to prevent any regression in the future.

Related to !112548 (merged)

MR acceptance checklist

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